Polymeric materials are influenced by electric: fields, which results in chemical changes such as crosslinking, chain-scission, the occurrence of double-bond formations, gas evolution and oxidation. To resist these degradation mechanisms, polymers are stabilized by additives. Electric insulating material is stabilized preferably by voltage stabilizers and antioxidants.
Voltage stabilizers, also known as "energy scavengers", protect the polymers against degradation by capturing electrons which function as carriers of energy in the electric field acting on the polymer. To prevent degradation of such an electrically insulating material for cables on the basis of polyethylene, it is known, for example from U.S. Pat. No. 3,629,110, to add voltage stabilizers in the form of aromatic hydrocarbon compounds such as phthalic acid anhydride, benzoguanamine, triphenyl formazan, quinoline, isoquinoline, indene, indole and quinaldine in contents of between 0.1 and 10 per cent by weight. According to this specification, the voltage stabilizer is also supplemented with an antioxidant, sterically inhibited thiophenol. Certain of the compounds such as quinoline and isoquinoline, which exhibit the greatest effect on preventing degradation of polyethylene as a result of electric load, also exhibit an undesired toxicity.
Oxidation in electric insulating materials is initiated and propagate preferably by means of radicals which are formed under the influence of electric fields. This degradation of the polymer is counteracted by the addition of an antioxidant, a radical scavenger, which reacts with the radicals and interrupts the degrading chain reactions. From EP-B 404 536, an antioxidant in the form of 5,10-dihydroindeno[1.2-b]indole, and substituted forms thereof, is known. The antioxidant is preferably intended to be included in medical preparations intended for treatment of diseases in which the biological tissue is broken down as a result of the influence of radicals. In addition, it is indicated that 5,10-dihydroindeno[1,2-b]indole can be used for checking and preventing degradation which is initiated and propagates by means of radicals in products such as skin care products and foodstuffs.
It is an object of the invention to provide an electric conductor which is provided with an electrically insulating layer in the form of a polyethylene, to which a combined antioxidant and voltage stabilizer has been added. It is a further object of the invention that this combined antioxidant and voltage stabilizer shall have a small toxicity to minimize the health hazards and the environmental influence when compounding the polyethylene, when manufacturing cable, and when taking care of cable scrap.